The present invention generally relates to fuel cell systems and, more particularly, to a fuel cell system that utilizes a pair of parallel turbines engaged to a compressor for increased system efficiency.
In a fuel cell system, power parasitesxe2x80x94such as compressors, fans, and pumpsxe2x80x94reduce the overall system efficiency and cause an increase in the fuel cell size and, therefore an increase in system cost. However, the stack size and cost can be reduced substantially if the system is designed to operate at elevated pressure. If the fuel cell system uses pressurized air, then a compressor-expander must often be used.
Important characteristics of the compressor-expander are its operating line, as well as its compressor and expander isentropic efficiencies. The operating line is defined as the dependence of the air pressure versus the airflow rate delivered to the fuel cell. As a general rule, a flat operating line is preferred where the pressure is constant at all operating points of the compressor-expander. The compressor and expander efficiencies are typically optimized at a selected design point, which is usually at maximum system power. But in doing so, compressor and expander efficiencies can be substantially reduced at part load because the compressor-expander operating point is far removed from the optimal design point.
A turbocompressor (or a turbocharger) is considered to be a preferred option for pressurization because of its high efficiency, relatively small volume and weight, and potentially low air contamination, if oil-less bearings are used. However, the characteristics of the turbocompressor (turbocharger) are often such that it provides air at a substantially lower pressure at part power system loads. Thus, it has a steep operating linexe2x80x94i.e., both compressor and turbine efficiencies can be substantially reduced at part loads. As a result, the lower pressure and the lower efficiencies lead to a lower overall system efficiency, in addition to an increased fuel cell stack size and cost.
In an effort to address the problem of operating at part load, U.S. Pat. No. 3,976,506 provides in a phosphoric acid system a compressor engaged to a single turbine. Waste energy in the form of hot pressurized gases in the system drives the compressor. At reduced loads, a portion of the compressed air from the compressor is bypassed around the fuel cell stack, is increased in temperature in an auxiliary burner where additional fuel is burned, and is flowed to the turbine to drive the compressor. The pressure of the air to the fuel cell stack is maintained while the amount of the air to the stack is reduced. Disadvantages to this design include the fact additional fuel must be burned to achieve the required compressor speed to maintain elevated pressure, which reduces the overall system efficiency.
In U.S. Pat. No. 4,041,210, a power plant for molten carbonate fuel cells includes a pair of two-wheel turbochargers are provided such that the compressor and turbine of one turbocharger are not engaged to the compressor and turbine of the other turbocharger. A portion of the fuel cell oxidant effluent is recycled while another portion drives the turbochargers. A recycle pump is driven by one of the turbochargers to assist in recycling. One disadvantage of this design, or any fuel cell system that includes one or more turbochargers or turbocompressors comprised of a single turbine engaged to one or more compressors, is that there is not a means to optimize the performance of each turbocharger or turbocompressor at part load. At part load, the turbine power will decrease due to both lower mass flow and reduced efficiency. Consequently, the power delivered to the compressor is reduced, thereby reducing system pressure.
A pair of turbines and a single compressor is provided in U.S. Pat. No. 3,982,962 wherein the two turbines are not engaged to one another. One turbine is engaged to the compressor and is driven by steam from a reactor burner that uses anode effluent from the fuel cell stack. The steam is condensed to liquid upon passing through the turbine, reconverted to steam by the fuel cell stack, and recycled to the turbine. The other turbine is air driven from anode and cathode effluent and can drive a generator as well as provide back-pressure to the power plant. Some of the disadvantages to this design are related to operation at part load. The performance of each turbine can not be optimized at part load, thus turbine power is reduced at part load due to both a reduction in mass flow and a reduction in turbine efficiency. The reduction in turbine power leads to reduced power to the compressor and a reduction in system pressure.
As can be seen, there is a need for an apparatus and method of providing a pressurized oxidant gas in a fuel cell power plant system. A further need is for an apparatus and method of providing pressurized oxidant gas in a fuel cell power plant system operating at reduced loads. An apparatus and method are also needed that increase the efficiency of a fuel cell power plant and, particularly a PEM fuel cell power plant, by providing substantially constant pressure at all operating points of the power plant. In other words, a flatter pressure operating line is needed for PEM fuel cell systems.
In one aspect of the present invention, a fuel cell system comprises a compressor; a fuel processor downstream of the compressor; a fuel cell stack in communication with the fuel processor and compressor; a combustor downsteam of the fuel cell stack; a first turbine downstream of the fuel processor; a second turbine downstream of the fuel processor and in parallel flow communication with the first turbine, and the second turbine being mechanically engaged to the compressor and first turbine; a bypass valve intermediate the compressor and second turbine, with the bypass valve enabling a compressed gas from the compressor to bypass the fuel processor; and a distribution valve in communication with the first and second turbines.
In another aspect of the present invention, a method of processing an oxidant gas and a fuel gas for a fuel cell stack comprises compressing the oxidant gas in a compressor to produce a compressed oxidant gas; flowing a first portion of the compressed oxidant gas to a fuel cell stack; flowing a second portion of the compressed oxidant gas to a fuel processor; flowing the fuel to the fuel processor to produce a hydrogen-rich gas; flowing the hydrogen-rich gas to the fuel cell stack; producing a heated oxidant gas from the fuel processor; splitting the heated oxidant gas into a first portion and a second portion of heated oxidant gas such that the first and second portions of heated oxidant gas flow parallel to one another; expanding the first portion of the heated oxidant gas in a first turbine to produce a first turbine exhaust; expanding the second portion of the heated oxidant gas in a second turbine to produce a second turbine exhaust; and mechanically engage the compressor, first turbine and second turbine to one another.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.